Digestive System
Structure, Absorption & Regulation
Master GI tract anatomy, nutrient absorption mechanisms, and hormonal regulation for MCAT Biological Foundations.
MCAT Bio/Biochem
The Alimentary Canal: A Continuous Tube
The gastrointestinal (GI) tract is a continuous muscular tube extending from the mouth to the anus. Each region has specialized structure and function. The four tissue layers: mucosa, submucosa, muscularis externa, and serosa: are present throughout but vary in composition by organ.
The GI Tract Pathway
Four Tissue Layers of the GI Wall
1. Mucosa
Epithelium + lamina propria + muscularis mucosae. Site of secretion and absorption.
2. Submucosa
Dense connective tissue with Meissner's plexus (submucosal plexus), blood vessels, and glands.
3. Muscularis
Inner circular + outer longitudinal smooth muscle. Contains Auerbach's plexus (myenteric). Drives peristalsis.
4. Serosa
Outermost connective tissue layer (visceral peritoneum). Reduces friction.
Stomach: Gastric Secretions
Parietal Cells
Secrete HCl and intrinsic factor. HCl activates pepsinogen and kills bacteria. \( \text{pH: } 1.5 \text{ to } 3.5 \).
Chief Cells
Secrete pepsinogen, which is cleaved by HCl to form pepsin (protein digestion).
G Cells
Secrete gastrin into the blood, stimulating parietal cells to produce more HCl.
Small Intestine: Three Segments
Absorption Mechanisms
1
Monosaccharides: Glucose and galactose via \( \text{Na}^+ \)-glucose cotransporter (SGLT1) on apical membrane, then GLUT2 on basolateral. Fructose via GLUT5 (facilitated diffusion).
2
Amino acids: \( \text{Na}^+ \)-dependent cotransport (similar to glucose). Di/tripeptides via PepT1 transporter (\( \text{H}^+ \) cotransport).
3
Fats: Bile salts emulsify fats into micelles. Free fatty acids and monoglycerides diffuse into enterocytes, are re-esterified into triglycerides, packaged into chylomicrons, and enter lacteals (lymph).
4
Water-soluble vitamins: Absorbed in small intestine. Vitamin B12 requires intrinsic factor from parietal cells and is absorbed in the ileum.
Fat Absorption: Micelles to Chylomicrons
Mechanical vs. Chemical Digestion
| Mechanical | Chemical | |
|---|---|---|
| Definition | Physical breakdown into smaller pieces | Enzymatic cleavage of covalent bonds |
| Mouth | Mastication (chewing) | Salivary amylase (starch), lingual lipase |
| Stomach | Churning / segmentation | Pepsin (protein), gastric lipase |
| Small Intestine | Segmentation contractions | Pancreatic enzymes, bile salts, brush border enzymes |
| Bond Changes? | No | Yes |
| Surface Area? | Increases | N/A (reduces polymer size) |
Hormonal Regulation of Digestion
Gastrin
Source: G cells (stomach antrum). Trigger: Peptides, distension, vagal input. Effect: Stimulates HCl and pepsinogen secretion, gastric motility.
Secretin
Source: S cells (duodenum). Trigger: Acidic chyme (low pH). Effect: Stimulates pancreatic bicarbonate, inhibits gastric acid.
CCK
Source: I cells (duodenum/jejunum). Trigger: Fats and amino acids. Effect: Stimulates pancreatic enzymes, gallbladder contraction, slows gastric emptying.
More GI Hormones
GIP
Source: K cells (duodenum/jejunum). Trigger: Glucose, fatty acids. Effect: Stimulates insulin release, inhibits gastric acid.
Somatostatin
Source: D cells (stomach, duodenum, pancreas). Effect: Inhibits gastrin, secretin, CCK, insulin, glucagon. The universal "off switch."
Neural Control of Digestion
1
Parasympathetic (vagus nerve): "Rest and digest." Increases motility, secretion, and blood flow to GI tract. Stimulates salivation, gastric acid, and pancreatic secretions.
2
Sympathetic: "Fight or flight." Decreases motility and secretion. Constricts sphincters and redirects blood away from the GI tract.
3
Enteric nervous system (ENS): "The gut brain." Can function independently. Meissner's plexus controls secretion; Auerbach's plexus controls motility.
How to Approach GI Questions
1
Identify the organ: Which part of the GI tract is being discussed? What are its specialized cell types?
2
Identify the nutrient class: Is it carbohydrate, protein, or fat? Each has distinct enzymes and absorption pathways.
3
Determine the signal: What hormone or nerve is regulating the process? What triggered its release?
4
Follow the transport: Where do absorbed molecules go? Portal vein (most nutrients) vs. lacteals/lymph (long-chain fats)?
5
Check feedback loops: Does the product inhibit or stimulate further secretion? Think of negative feedback via somatostatin or low pH inhibiting gastrin.
Worked Example: Starch Digestion
Trace the complete digestion of a starch molecule from mouth to absorption:
1
Mouth: Salivary amylase breaks starch into maltose and dextrins. Optimal \( \text{pH: } \sim 7 \).
2
Stomach: Amylase is inactivated by low pH. No significant carbohydrate digestion occurs here.
3
Duodenum: Pancreatic amylase continues starch breakdown. Brush border enzymes (maltase, isomaltase, sucrase, lactase) produce monosaccharides.
4
Absorption: Glucose enters enterocytes via SGLT1 (apical), exits via GLUT2 (basolateral) into portal blood → liver.
Worked Example: Protein Digestion
1
Stomach: HCl denatures proteins. Pepsin (from pepsinogen) cleaves proteins into large polypeptides. Pepsin works at \( \text{pH } 1.5 \text{ to } 3.5 \).
2
Duodenum: CCK stimulates release of pancreatic proteases: trypsinogen (activated by enterokinase to trypsin), chymotrypsinogen, proelastase, procarboxypeptidase.
3
Activation cascade: Enterokinase activates trypsinogen → trypsin, which then activates all other zymogens.
4
Brush border: Aminopeptidases and dipeptidases yield individual amino acids. Absorbed via \( \text{Na}^+ \)-dependent cotransport into portal blood.
Common Mistakes
✕ Fat goes to portal vein
Long-chain fats are packaged into chylomicrons and enter lacteals (lymph), not the hepatic portal vein. Only short/medium-chain fatty acids go directly to portal blood.
✕ Confusing secretin and CCK
Secretin: triggered by acid, stimulates bicarbonate. CCK: triggered by fats/amino acids, stimulates enzymes and bile. Different triggers, different effects.
✕ Pepsin works everywhere
Pepsin is only active at low pH (\( 1.5 \text{ to } 3.5 \)). In the duodenum, pH is neutralized by bicarbonate, so pepsin is irreversibly inactivated.
Enzyme Summary Table
| Enzyme | Source | Substrate | Product |
|---|---|---|---|
| Salivary amylase | Salivary glands | Starch | Maltose, dextrins |
| Pepsin | Chief cells (stomach) | Proteins | Polypeptides |
| Trypsin | Pancreas | Polypeptides | Smaller peptides |
| Lipase | Pancreas | Triglycerides | \( \text{FFA} + \text{monoglycerides} \) |
| Lactase | Brush border | Lactose | Glucose + galactose |
| Enterokinase | Brush border | Trypsinogen | Trypsin (active) |
Question 1 of 5
A patient has a tumor that destroys the parietal cells of the stomach. Which of the following consequences is most likely?
Parietal cells secrete both HCl and intrinsic factor. Intrinsic factor is required for vitamin B12 absorption in the ileum. Without it, the patient develops pernicious anemia. Gastric lipase comes from chief cells, bicarbonate from the pancreas, and enterokinase activates trypsinogen.
Question 2 of 5
Which hormone is released in response to acidic chyme entering the duodenum and stimulates the pancreas to secrete bicarbonate?
Secretin is released by S cells of the duodenum when pH drops (acidic chyme arrives). Its primary action is to stimulate pancreatic bicarbonate secretion to neutralize the acid. CCK is triggered by fats/amino acids and stimulates enzymes, not bicarbonate.
Question 3 of 5
Long-chain fatty acids are absorbed into enterocytes and ultimately enter the circulation via which route?
Long-chain fatty acids are re-esterified into triglycerides inside enterocytes, packaged into chylomicrons, and released by exocytosis into lacteals (lymphatic vessels). They travel via the thoracic duct to the left subclavian vein. Only short/medium-chain fatty acids enter the portal vein directly.
Question 4 of 5
A researcher observes that enterokinase activity is blocked in the duodenal brush border. Which process is most directly impaired?
Enterokinase converts trypsinogen to trypsin, which then activates chymotrypsinogen, proelastase, and procarboxypeptidase. Without enterokinase, the entire pancreatic protease cascade fails. Fat emulsification (bile salts), bicarbonate (secretin-mediated), and glucose absorption are unaffected.
Question 5 of 5
Which nervous system component controls GI motility and is located between the circular and longitudinal muscle layers?
Auerbach's (myenteric) plexus is located between the inner circular and outer longitudinal smooth muscle layers of the muscularis externa. It primarily controls motility (peristalsis and segmentation). Meissner's plexus is in the submucosa and controls secretion. The vagus nerve provides extrinsic parasympathetic input but is not located within the muscle layers.